The oh-so-satisfying pop that results from cracking your knuckles is more than just a great way to occupy nervous hands or let your enemies know they're crusin' for a bruisin'—it's also a hotly contested medical mystery. While it has been pretty well documented that cracking you knuckles doesn't increase your risk for arthritis (a popular misconception), researchers are still baffled by just what might account for the popping sound itself.
In recent years, two competing theories have emerged: one which claims that the popping is the result of collapsing air bubbles in the joints, the other claiming that it is the result of air bubbles forming in the joints. The knuckle crack debate is still unresolved, but a study presented this week at the Radiological Society of North America's annual conference may have complicated things a bit when the researchers showed that a small flash occurs along with the popping sound.
By studying 40 individuals—30 with a history of knuckle cracking and 10 without—the research team used ultrasound to film 400 knuckle pulls, 62 of which produced audible popping. But it was what seemed to happen after the popping which left the researcher's baffled.
"What we saw was a bright flash on ultrasound, like a firework exploding in the joint," said Dr. Robert Boutin, a professor of radiology at University of California, Davis and the lead author of the study. "It was quite an unexpected finding."
The team's study comes on the heels of another knuckle cracking study published in PLOS One in April, which seemed to resolve the mystery of the knuckle pop in favor of bubble formation in the joints, but made no mention of the mysterious flash documented by Boutin and his colleagues.
"There have been several theories over the years and a fair amount of controversy about what's happening in the joint when it cracks," Boutin said. "We're confident that the cracking sound and bright flash on ultrasound are related to the dynamic changes in pressure associated with a gas bubble in the joint."
Although the video appears to show the flash of light coming after the popping sound, the team said they will need to do more observations to determine the order of events. For now, one of the few things that the team can say for certain is they observed no immediate pain or decrease in grip strength from the participants who were able to crack their knuckles.
"We found that there was no immediate disability in the knuckle crackers in our study," Boutin said. "Although further research will need to be done to assess any long-term hazard—or benefit—of knuckle cracking."